Endogenous regeneration of the thymus is a crucial function that allows for renewal of immune competence following infection, shock or common cancer therapies such as cytoreductive chemo- or radiation therapy. Thymic regenerative capacity diminishes with age and remains a poorly understood area. This is particularly relevant for recipients of allogeneic hematopoietic stem cell transplantation (allo-HSCT), who experience prolonged post-transplant T cell deficiency caused by cytoreductive conditioning and graft-versus- host disease (GVHD), which results in increased morbidity and mortality from infections and malignant relapse. Therefore, the main goal of this proposal is to understand the proceses underlying endogenous thymic regeneration so that they may be exploited into clinically relevant strategies for immune rejuvenation. Interleukin-22 (IL-22) is a recently described cytokine that is produced by T-helper (Th)-17 cells and innate lymphoid cells (ILCs) and promotes innate immunity and homeostasis of epithelial cells in the intestines, lung and skin. We have recently revealed a network of thymic regeneration centred on IL-22 and triggered by the depletion of CD4+CD8+ double positive (DP) developing thymocytes. Although at baseline IL-22 deficiency was redundant for normal thymopoiesis, thymic recovery was impaired in IL-22-deficient mice and intrathymic levels of IL-22 were significantly increased in wildtype mice following thymic injury. IL-22, which signalled through thymic epithelial cells (TECs) and promoted their proliferation and survival, was upregulated by radio- resistant innate lymphoid cells (ILCs) after thymic injury in an IL-23 dependent manner. Importantly, administration of recombinant IL-22 enhanced repair folowing thymic injury. These findings highlight an endogenous network of thymus regeneration whereby 1) the depletion of DP thymocytes triggers 2) upregulation of IL-23 by DCs, which induces 3) the production of IL-22 by thymic ILCs. This cascade of events leads to regeneration of the supporting epithelial microenvironment and, ultimately, to rejuvenation of thymopoiesis. Based on these findings, we hypothesize that (a) IL-22-mediated regeneration will be translated into functionally enhanced T cell reconstitution, (b) endogenous regenerative pathways in the thymus are triggered by a damage-sensing mechanism intrinsically tied to the loss of DP thymocytes and a negative signal they typically produce, (c) I-22 directly promotes the proliferation of TECs but also enhances their capacity to respond to additional trophic factors, and (d) breakdown of IL-22 regenerative pathways underlie chronic thymic involution, including age-related atrophy. Exploring the functional translation of IL-22-mediated regeneration is a natural progression of my postdoctoral work and will constitute the K99 mentored phase of this proposal. A logical extension of these studies, exploring the underlying mechanisms and implications of the IL-22 regenerative pathway, will be explored in the independent R00 phase of this proposal. The mechanistic and pre-clinical studies outlined in this proposal have the potential to define an important novel pathway in thymic regeneration, which could result in clinical approaches to enhance T cell immunity, not only for recipients of allo-HSCT, but also for individuals with T cell deficiencies due to aging (lymphoid atrophy), autoimmune diseases, genetic causes (such as SCID), infectious diseases (HIV, hepatitis B/C), corticosteroids, shock, radio- or chemo-therapy and radiation injury (nuclear accident or terrorism).